The present invention relates to an optical amplifier, and particularly to an optical amplifier used suitably for systems which adopt the optical transmission system based on wavelength division multiplex.
Recently, optical transmission systems which use optical amplifiers as repeaters are studied and put into practice vigorously. It is particularly beneficial for the multi-media service industry typified by the Internet to increase the communication capacity based on the WDM (Wavelength Division Multiplex) system for multiplexing individual signal lights of different wavelengths. In the optical communication system using the WDM technique, the optical repeating amplifier which amplifies all individual signal lights at once has an extremely crucial role for the extension of transmission distance.
For the optical amplifying medium which forms the optical amplifier, optical fiber, with rare earth being added thereto, is useful, and it is under study and about to be put into practice. Particularly, erbium-doped fiber (hereinafter, abbreviated as xe2x80x9cEDFxe2x80x9d), which is active for amplification in a wide range of wavelength in which the loss of optical fiber is small, is used extensively in optical fiber communication systems.
In order for the optical amplifying medium to be active for amplification in the band of signal light wavelengths, a pumping light which is shorter in wavelength than the signal light is inputted together with the signal light into the medium. The optical amplifying medium has at its end the connection of a WDM optical coupler so that the signal light and pumping light are incident efficiently to the medium.
However, the rare-earth-added optical fiber is narrow in its gain flatness region against signal wavelengths in amplifying the WDM signal light at once, and even with the imposition of limitation of the wavelength range, the gain flatness against signal wavelengths is deteriorated by the variation of input signal light power.
As a scheme of overcoming this problem, there is known a technique for suppressing the wavelength dependency of the gain at different input light power levels, in which the input power and output power of the optical amplifying medium are measured and the pumping light power is adjusted based on automatic gain control (AGC) so that the average gain evaluated from the measured power levels is constant. The optical amplifier based on this scheme further adopts constant output control by use of a variable optical attenuator.
However, the actual optical amplifier has its gain characteristics varied by the temperature variation in addition to the variation of input light power. Therefore, an optical amplifier, which is designed to have a minimal gain""s wavelength dependency at a certain temperature, cannot be rid of the wavelength dependency based solely on the AGC due to the variation of operating temperature.
With the intention of overcoming this problem, there is known a technique of flattening the gain of optical amplifier throughout the wavelengths. For example, a technique disclosed in Japanese Patent Laid-open (Kokai) No. Hei 4-11794 implements constant temperature control for the optical amplifying medium by using a temperature control device such as a Peltier device, and the optical amplifier has a flat gain throughout the wavelengths. Another technique disclosed in Japanese Patent Laid-open (Kokai) No. Hei 10-335722 is designed to control the characteristics of the optical filter device in response to the temperature measured with a thermistor, thereby offsetting the gain""s wavelength dependency.
However, the above-mentioned conventional techniques involve the following problems. A system which uses a temperature control device such as a Peltier device has inevitably an increased power consumption, and the system will be complex and bulky due to the need of heat dissipation of the Peltier device. A system which offsets the gain""s wavelength dependency by using a thermistor and optical filter device encounters the difficulty in the availability of an optical filter device capable of rectifying sufficiently the gain""s wavelength dependency of EDF attributable to the variation of input light power and temperature. Moreover, this system having the additional control parameter of temperature will inevitably have an increased component parts and intricate control operation.
A primary object of the present invention is to overcome the foregoing prior art deficiencies, and specifically provide an optical amplifier which is smaller in power consumption and number of component parts, simpler in structure, and rid of the gain""s wavelength dependency against the variation of input light power and temperature.
A second object of the present invention is to provide an optical amplifier which has a flatter gain (smaller in dependency of gain on the input signal power, temperature of optical amplifier and input signal wavelengths).
A third object of the present invention is to offer a structure of optical amplifier which is smaller in power consumption than conventional counterparts.
A fourth object of the present invention is to provide an optical amplifier which has a smaller number of component parts as compared with conventional counterparts.
The inventors of the present invention have conducted experiments on rare-earth-added fiber as a kind of optical amplifier, and found the following facts. (1) The output light power vs. pumping light power proper curve, with the input light power and temperature being varied and with the gain""s wavelength dependency being retained virtually constant, i.e., the gain tilt is virtually constant throughout the wavelengths, is a virtually simple increase function or a virtually linear function. (2) If the output light power and pumping light power are related to lie on the above-mentioned proper curve, gain characteristics with a virtually invariable gain""s wavelength dependency against the variation of input light power and temperature can be obtained.
Accordingly, based on the use of a proper curve without or with practically a sufficiently small gain tilt and on the retention of the output light power and pumping light power on the proper curve, it is possible to accomplish an optical amplifier which operates virtually on the proper curve without or with practically a sufficiently small gain tilt, as will be explained in detail later.
In case the input WDM signal has a gain tilt, a proper curve of the opposite tilt for the input WDM signal is used so that the output light power and pumping light power are related to lie on the proper curve, and the EDF optical amplifier can produce an output light without a gain tilt.
The present invention is based on the foregoing findings, and the above-mentioned objectives are attained by the provision of a characteristic information table which contains data of the output light power vs. pumping light power proper curve of the optical amplifying medium at a virtually constant gain tilt of WDM signal light, and the provision of a pumping light controller which controls the pumping light power so that the output light power and pumping light power of the optical amplifying medium are related to lie virtually on the given proper curve. Adopting this scheme retains the relation of the output light power and pumping light power virtually on the proper curve, and attains gain characteristics without wavelength dependency or with practically a sufficiently small gain tilt against the variation of input light power and ambient temperature.
The pumping light controller is arranged, for example, as follows. The proper curve is obtained in advance. The relation between the pumping power and output power on condition that the gain tilt is virtually minimum, i.e., the variation of output power in response to the increase of pumping power, can be evaluated based on the experiment. The output power vs. pumping power relation in terms of an approximate curve provides the proper curve. Light detecting means are used to detect the pumping light power value and the output light power value.
A means which inputs both detected light power values, takes out a pumping power value based on the proper curve in response to the input of the detected output light power value, and makes the detected pumping light power value the taken out pumping light power value is part or whole of constituent parts of the pumping light controller. The controller can be formed of an operational amplifier having its two input terminals. The detected pumping light power value is inputted to one of the input terminals and the taken out pumping light power value is inputted to the other input terminal so that a light source of pumping light is controlled with the output signal of the operational amplifier.
Specifically, a first optical amplifier based on this invention comprises an optical amplifying medium which amplifies an input light by receiving a pumping light, a light source which generates the pumping light, and a pumping light controller which controls the pumping light power. The pumping light controller includes a characteristic information table which contains data of the relation between the output light power and pumping light power of the optical amplifying medium at a virtually constant gain tilt among wavelengths of the input light, and a light source controller which controls the light source by using the pumping light power value which is read out of the table in response to the output light power level resulting from the amplification of the optical amplifying medium so that the pumping light power reaches the readout pumping light power.
The input light can possibly have its power level varied inherently even if it does not have wavelength dependency. An effective scheme to deal with this matter is the provision of a variable optical attenuator for the optical amplifier, with the degree of attenuation being controlled so that the output light power is virtually constant.
A second optical amplifier based on this invention, which is derived from the first optical amplifier and intended to have a constant output light power, further includes a variable optical attenuator which attenuates the output light of the optical amplifying medium, and an attenuation controller which controls the degree of attenuation of the attenuator by using the measurement result of output light power of the attenuator so that the attenuator output light power has a certain level.
A third optical amplifier based on this invention, which is derived from the first optical amplifier and intended to have a virtually constant output light power, further includes a variable optical attenuator which attenuates the output light of the optical amplifying medium, and an attenuation controller which controls the degree of attenuation of the attenuator by using the measurement result of output light power of the optical amplifying medium so that the optical attenuator has an output light of a certain power level.
The second and third optical amplifiers need to have their output light powers stepped up initially by the quantity of attenuation by the optical attenuator. In case the optical amplifier drives an optical fiber cable, a large output light power which is accompanied by a large power consumption is required, and the above-mentioned stepped-up output light power preparatory to attenuation can be a matter of power capacity. An effective scheme to deal with this matter is to form the optical amplifier in two-stage amplifiers, with the attenuator being placed between these amplifiers. The attenuation of output power is not significant to the former amplifier.
A fourth optical amplifier based on this invention, which is derived from the above-mentioned two-stage where the optical amplifier is defined as a optical amplifying unit, includes two optical amplifying units connected by being interposed by a variable optical attenuator, and an attenuation controller which is connected to the output of the latter optical amplifying unit and adapted to control the degree of attenuation of the output light of the latter optical amplifying unit by using the measurement result of the output light power of the latter optical amplifying unit so that the latter optical amplifying unit has an output light of a predetermined power level.
The fourth optical amplifier has a predetermined output light power, which allows to determine the pumping light power of the latter optical amplifying unit fixedly.
A fifth optical amplifier based on this invention is intended to make the pumping light power value read out of the table of the latter optical amplifying unit correspondent to a predetermined output power, thereby fixing the pumping light power of the latter optical amplifying unit.
In case the input light has a relatively large power level, the former optical amplifying unit can be eliminated.
A sixth optical amplifier based on this invention is made up of the latter optical amplifying unit solely and the optical attenuator of the fifth optical amplifier.
Optical transmission systems are required to have high amplification factors in some cases (e.g., a system having a long segment of fiber cable between repeaters). An effective scheme to deal with this case is the installation of multistage optical amplifying units.
A seventh optical amplifier based on this invention is derived from the fourth optical amplifier, with its latter stage being made up of amplifying units of n in number, thereby raising the amplification factor.
The seventh optical amplifier has a predetermined power level of the latter-stage output light, allowing the pumping light power of each of latter optical amplifying units to be determined fixedly.
An eighth optical amplifier based on this invention is derived from the seventh optical amplifier, with its latter stage made up of optical amplifying mediums of n in number which are connected in series and each of which amplifies an input light by inputting a pumping light, light sources of n in number which generate pumping lights to be fed to the n optical amplifying mediums, and a pumping light controller which controls the n pumping lights to have power levels which are expressed in terms of predetermined constants xcex11, xcex12, . . . , xcex1n and a reference pumping power Pp to be xcex11Pp, xcex12Pp, . . . , xcex1nPp. The pumping light controller includes a characteristic information table which contains data of the output power levels vs. reference pumping power level Pp characteristics of the last-stage optical amplifying mediums at a constant gain tilt throughout the input light wavelengths, and a light source controller which controls the n light sources by using the reference pumping power value Pp which is read out of the table in response to a predetermined output power value so that the n light sources have pumping light power levels which are equal to the reference pumping light power Pp multiplied by the above-mentioned constants.
The foregoing inventive optical amplifiers are advantageous in that the gain""s wavelength dependency against the variation of input power and temperature can be made smaller, the characteristic information table can be formed of a semiconductor IC ROM (Read Only Memory) for example, and the controller can be formed of a semiconductor IC processor, whereby these optical amplifiers can be smaller in power consumption and number of component parts and simpler in structure.